1. Field of the Invention
The present invention relates to an improved solar collector, and more in particular a special solar collector plate, a method for manufacturing such a plate, and a method for safeguarding the operation/performance of a solar collector plate.
2. Description of the Related Art
A solar collector is a device that converts energy in solar radiation, to heat at a utilizable temperature. The energy conversion takes place in an absorber. This absorber is designed in such a manner that the radiation is absorbed and converted to thermal energy, and the energy is transferred to a heat-carrying medium that transports it away from the solar collector plate absorber and into a heat store, or to immediate utilization. The heat-carrying medium may be a gas, e.g. air, or a liquid, e.g. water. The solar collector plate is usually insulated, and on the absorber side facing the sun, one will often use insulation that is transparent to sunlight, for instance glass or transparent plastics, and on the plate side facing away from the sun, one may use mineral wool or some other solid insulation material that is temperature resistant. A flat solar collector has a cover plate, and most often an absorber that is flat, i.e. there is no focusing of sunlight in toward the absorber. Flat solar collectors without a cover plate in front of the absorber, are also in use.
The present invention concerns a solar collector plate with absorber in a flat solar collector. Most flat solar collectors use absorbers manufactured from a metal, however absorbers in plastic materials can also be found in the market. Most plastic materials used in solar collectors have a softening temperature that is too low to make them suitable in flat solar collectors with a cover plate, Several types of plastic solar collectors without cover plate are used for low temperature purposes, for instance heating of swimming pools. However, some absorbers are manufactured in plastic materials that can withstand temperatures as high as 150-160° C., and these absorbers are suitable also for solar collectors with an insulating cover plate.
The reason for using a plastic material in the absorber, is primarily favourable large-scale manufacturing costs in comparison with metal absorbers. The manufacturing cost is a critical parameter, since the use of solar energy is primarily dictated by whether it may turn out to be competitive regarding price, compared to the more conventional energy sources/carriers. However, plastic materials that are able to satisfy the temperature requirements in a solar collector with a cover plate, are relatively costly, and therefore competitiveness presupposes a design that is able to reduce the use of plastic material per unit area to a minimum. The capability of the plastic materials regarding withstanding the strains to which an absorber in a flat solar collector is subjected, puts restrictions on the combination of mechanical, thermal and radiation-related influences. The less plastic material used per unit area, the stricter these requirements will be.
One type of mechanical strain for which the plastic materials are vulnerable, especially in combination with a high temperature, arises if there are pressurized channels or cavities inside the solar collector plate.
Since solar collectors at northern latitudes are often arranged standing/slanting, and the coolant is often brought up to a certain height by means of pumping, is pressures will usually arise, and in some cases there will appear rapid pressure changes, for instance when shutting down or starting up, which pressure changes may be harmful to the solar collector or to the whole liquid system. The main problem that the present invention tries to solve, is to equalize harmful pressure transients and kickbacks.
Liquid filling of the channels is necessary with regard to heat transfer, since the plastic material has a very small coefficient of thermal conductivity (λ=0.1−0.3 W/m deg). When the liquid gets in direct contact with the underside of the thin plastic surface where the radiation energy is deposited as heat, the necessary transport path for heat through the plastic material is minimized.
However, the hydrostatic pressure may then become large inside the channels.
In Norwegian patent publication no. 179925 in the name John Rekstad, it is disclosed how particles of a certain size, arranged in the liquid-conducting channels in a plastic solar collector plate, create capillary forces that make it possible to establish liquid-filled solar collector plates without simultaneously building up a liquid pressure inside the liquid-conducting channels.
However, the granulates in the channels of the solar collector plate entail production technical disadvantages that also lead to cost-related consequences. They also contribute to increased weight, which causes additional strain on the plastic material, and is a drawback in assembling and transporting. If the granulates are manufactured from another material than the rest of the solar collector plate, they must be removed prior to a possible material recovery when the plate shall be destructed. Recyclability is today an important characteristic when judging solar collectors.
One further problem solved by the present invention, is eliminating the granulate, but nevertheless making it possible to utilize a capillary effect in the channels in order to diminish pressure problems.